Answers

Assuming that the border is in sun, try Heleniums, Asters (Michaelmas daisies), Rudbeckia 'Goldsturm' (very reliable and easy), Shizostylis (a bulb), grasses for their airy flowers. There are also some Kniphofias (red-hot pokers) that flower later. Dahlias will also flower until the frost. For a moe shady border try Japanese Anemones.

if your question is about osteospermum then it may flower into the autumn but i dont find it fully hardy where i am. i would take cuttings and overwinter it in a frost free place just in case you lose the parent plant.

I find Verbena Bonariensis start flowering around July but go on and on and on till November, i still have the tall stems in now for interest.

They are actually borderline hardy but it depends what region you live in, great self seeders too, smell beautiful, graceful tall and airy, grows upwards on square wiry stems, if lost to the winter don't worry they will self seed all over the place.